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PCBD-Oct2017

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October 2017 • The PCB Design Magazine 15 SIGNAL INTEGRITY: THE EXPERTS WEIGH IN copper over the whole board, which is the second difference. This makes for very consis- tent copper thickness over the whole panel and in the hole as well. And the third major dif- ference is our positive-acting electrodeposited photoresist, where the areas exposed to UV light develop off. Then we etch in cupric chloride. In this way, we get very uniform con- ductors, and that's how we can control impedance to close to 2%." "The main issue on the board fabrication side is con- sistent lamination, making sure that the board shop has a good, consistent way of mea- suring at different weaves and resin contents," Sunny explained. "If a PCB manufacturer is pulse plating, the consistency will be pretty good, but that's generally one of the areas where most will have difficulty—with copper plating and cop- per etching as well. So, people with more direct imaging and pulse plating technology, or our way of doing things, will get you finer imped- ance characteristics. But a lot of it comes back to the engineers themselves, discussing stackups and what they want in the end with their PCB supplier—having that thorough conversation with the engineer on the board side to ensure that what they want is what they're going to get with the stackup. Those are the two main things: The process has to be controlled and consistent, and there has to be a clear commu- nication with the design engineers." Mark Thompson had a similar viewpoint. "I agree with you guys as far as the negotiation process. If a customer approaches you before they have a design ready, and their engineer has just told them, 'I have 8-, 10-, and 12-layer boards, and I need to get a slew of impedances from you, and a dielectric stackup with effective dielectric constants for each of the subsections,' there are several questions that must be asked at that time. A lot of us come up with an imped- ance checklist that asks all those basics, such as: What's the material type? What's the cop- that whole discussion of weave effect, for two reasons. One of which was simply the emo- tional tone that got tied to the whole subject. I remember one phrase in particular from one particular author. In the mid- dle of a DesignCon paper he said, 'Be afraid, be very afraid.' I'm sorry, but that phrase has no place in a professional dis- cussion. In 2012, I wrote an online article that said, 'Oh, by the way, you can accept a certain amount of skew. You get a little bit of degradation but as long as you keep the skew within like an eighth of a wavelength at the maximum frequency of interest, the ef- fects can be minimal.' "Then a couple years later, Istvan Novak and his people at Oracle came in with a DesignCon paper where they actually tested in the lab what the acceptable level of skew was. Again, this is an equation that has not only a left-hand side but a right-hand side. So don't just take this 'ooh, scary' point of view. This is an engineer- ing problem; let's work this as an engineering problem. There's a tolerance we can live with." From the Fabricator's Viewpoint Sunny Patel said, "We make boards for dif- ferent technologies, and we can control the im- pedance to nearly 2%. We can talk about the challenges with keeping those tolerances that tight, like how the dielectric thickness and the etching characteristics in plating are really the key parts of the fabrication process." "We know that most manufacturers control impedance close to 10% most of the time," add- ed Yogen. "To control to under 5%, they charge so much extra money. There are three main dif- ferences between the conventional process, as we call it, and our process. First, our press is resis- tive-heating so the outer layer copper becomes a heater, and that's why we don't have to press at 350 PSI—we use 230 PSI—so the dielectric is very consistent across the whole panel. And then we drill, clean the holes, and panel plate Sunny Patel

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